Hmm, maybe I am confused as well, but wouldn't a wetland be quickly overloaded when flooding it with high strength settled sludge (which is what sludge drying beds are designed for)?

Edit: Well... strictly speaking septage should have a similar strength as regular waste-water, but in most cases septic tanks are not emptied before there is a large sludge accumulation as the supernatant waste water/"effluent" is continuously discharged to a solids free sewer, secondary on-site treatment or is illegally infiltrated to the same effect.

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Featured UserMay 2015

Drear All
I got information from the book Faecal Sludge Management by EAWAG/IWA published in 2014. I am thankful to Florian for his suggestion to go through the book. There are other books/ reports on different aspects of FSM published by EAWAG/SANDEC.
Kevin has rightly mention regarding loading of 100-200 kg TS /m2 of filter bed. There is also suggestion that there should be 200 mm of FSM load in filter bed. It appears to be more practical. Under different books/ reports / papers there are some differences in data and design parameters, for Filter bed and Constructed wetlands. This could be due to local environmental conditions, characteristics of FSM etc.
Removal of pathogens from leachate through sand filter is mainly due to physical barrier. I don’t have any evidence in this regard as sought by HAP. If anybody has evidence, may like to provide.
Constructed wetland is also one of the options for FSM. The problem is safe removal and reuse of biosolids. HAP may like to inform the process of removal of biosoilds based on his experience.
Regards

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Featured UserDec 2015

Dear Krishan, dear Pawan,

If I am talking of constructed wetlands for sludge disposal, I am talking about vertical flow wetlands which are constructed similar to sand drying beds only that they are planted with swamp plants (macrophytes) or at least water tolerant plants and care is taken that they don't ever get completely dry (even though that is sometimes difficult to achieve). These wetlands will usually have a large freeboard above the planted surface where the sludge can accumulate (sometimes more than one meter high in height). As the wetland is used, the solids will accumulate in that freeboard for years and slowly fill it up. Because of the time involved, the solids will also degrade and minaralize, so that the accumulation is actually much less than the actual volume of deposited solids. And the plants will grow into these solids and turn it into their rooted zone. That explains that the permeability of the layer is very good and loading can be high. So, what you actually end up in the freeboard is an accumulation of humus with plants growing in it.

When it comes to emptying a wetland, usually after years of operation, you take it out of operation for 6 months (something, of course, you have to plan for in the design of the facility), so that the pathogens are going to die due to the usually intense sunlight and the lack of nutrients, and then you can empty what was originally the freeboard above the original surface, then you plant the plants back into a remaining layer of humus, and start loading again. I'd call that very efficient!

In the case of Adjumani, which is a small (35 000 inhabitants) country town in northern Uganda, we had three wetlands of (originally) about 20 m3 capacity. One was meant as a back up, the other two could take at least 8 loads per day from one truck that would be operating in the area. After some days, the truck would go to a different area, and the wetlands could be resting for weeks and even months until another truck comes along. As I was saying, we've never seen the limit of what the wetlands can take, and it's only now that one of the basins is getting full.

Concerning the effluents from the wetlands - little was actually coming out, most of it was evaporated, and what was coming out was seeping into an adjacent land somewhat larger than the wetlands themselves. There, agricultural usage would have been possible, for example growing of bananas or papayas, but because of security reasons and the intrusion of cattle, this has never been successful - the fence of the facility got torn down by the local shepherds who apparently thought it nice to graze their animals on the beautifully green wetland areas - some of the many problems we had at that facility. Technically, the facility has far exceeded our expectations.

I hope that has given you some explanations and insight.

Kind regards,

H-A

PS: At the moment, I am working on a Susana case study on the sludge treatment in Adjumani, which will be giving more details.

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Dear all,
only now I got right what pawan wants do to and I can only agree with Hans. I am using a constructed wetland (we call it sludge mineralization bed) since 8 years with about 150 kg dry solids/a (rough estimation as no real data) without any problems and it is still not full. Volume reduces VERY largly. Important is:

exchange between beds and longer resting periods

do not overload the beds - really a danger as they seem somewhat too large in the beginning.

Really as pointed out by otheres the daily hydraulic loading is not the parameter it is the yearly sludge load.
When designing you should start with small beds to get experience as there are plenty of bad examples out there, using some plants and claiming to be sludge mineralization beds.

Much less hazzle to maintain as sludge drying beds, qhich you have to clean out several times per year - that is the reason people keep on loading on an on until they colapse.

This is an important discussion. Thank you, Hanns-Andre, for this great description of a successful example.

I would like to add my voice to the suggestion that plants be included, for the following reasons:
<> The roots of the plants will help to maintain the permeability of the bed, will likely add oxygen (depending on the species of plant), and will generally encourage the development of a healthy community of soil microbes.
<> The plants will block the movement of air over the sewage/septage and thus greatly reduce smell.
<> A significant amount of water will be transpired into the air via the plants.
<> The grass produced is an important as forage for animals (which could be safely grazed after an adequate drying period, especially right before the soil is emptied from the bed).

In a situation like this, would it be worthwhile to have a UASB (Upflow Anaerobic Sludge Blanket) or an ABR (Anaerobic Baffled Reactor) to generate biogas and then treat the water via subsurface wetlands? Would the irregular flow of septage and the higher cost preclude this? Would the biogas pay for this?

Am I right that Christoph meant 20cm and not 20mm of sand?
Would this sand be on top of a network of perforated pipes for drainage of the water (as described in the Eawag Compendium)?
Or is the water only expected to evaporate or soak into the soil?
This water could be further treated via subsurface wetlands or could irrigate fruit trees, etc., via buried perforated hoses.

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Featured UserDec 2015

Hello Chris,

Thanks for your post and the points you are raising. What you are calling elephant grass is not at all what is known as elephant grass here in northern Uganda (and it seems it's the same in Adjumani and Moroto Districts, where I have been stationed). There are probably several species known as 'elephant grass', but what we know as such (and Ugandans should be competent about that) is the dominant species in the savannah.

As it turns out, this Ugandan elephant grass also grows in areas that are temporarily swampy during the rainy season. And it's the species that has taken over in two of the three basins in the facility in Adjumani. It's the two that are used predominantly, but the third one is also used sometimes, especially if one of the other two is out of order for some reason. We had originally planted a variety of sedges that was growing in an adjacent wetland, but they have all died because the basins got too dry during the dry season. In the third basin a type of grass took over that we have originally planted in order to stabilize the clay slopes of the basins. We have made no attempt to try to change the vegetation that is developing naturally, except that trees and shrubs are removed.

The basins are made of clayish loam and constructed in an area where that type of material was making up the soil (so that the in-situ loam was used as the bottom liner). Above we have the following layers: medium gravel: 100 mm, fine gravel: 100 mm, course sand: 300 mm, top soil: 100 mm (where the plants were planted). The drainage pipes are located in trenches below the gravel layers that act as drainage layers.

As I was mentioning, we are just allowing the effluents to seep into an adjacent land where the top soil that has been removed for the construction of the basins and the access road has been spread. Hence it is extremely fertile land, and the original idea was to use it for cultivation (which hasn't been implemented yet for the reasons stated in my last post).

In the third basin, we tried 150 mm of course sand and 150 mm of fine sand instead of the course sand only. So far, there has not been any visible operational difference except that the grass cover is different. Of course, the hope is that a major portion of the pathogens are withheld in the basins, and in that regard, the layer of fine sand may be more effective. Unfortunately, we never had the opportunity to make analyses in that regard.

Concerning biogas, I don't think it's an option, certainly not in Adjumani. There isn't the required consistent supply of sludge, and there wouldn't be any market for the gas in the area since the facility is located out of town.

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Dear all,

let me be more specific about the species of grass that got hold of the constructed wetlands in Adjumani. The first one, locally called 'elephant grass', according to the literature should be a Hyparrhenia species, see, for example: en.wikipedia.org/wiki/Hyparrhenia_rufa
You can see in the description that this species is also growing in seasonally flooded grasslands. At the same time, it is highly resistant to drought, so it should be an ideal species where the supply of sludge is varying strongly or is intermittent.

The second species in the third basin would have to be crosschecked, but I think it is a species that is locally used for lawns. It doesn't become very high, but it is very aggressive in growth.

As for the 'elephant grass' mentioned by Chris, Pennisetum purpureum, it is a species that is dominant in regions with higher rainfall in southern Uganda. Apparently, it is the one called 'elephant grass' by botanists, and it's also known as Napier grass.

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Featured UserMay 2015

Dear Hanns-Andre

The following 2 points are quoted from your post:

1. One was meant as a back up, the other two could take at least 8 loads per day from one truck that would be operating in the area. After some days, the truck would go to a different area, and the wetlands could be resting for weeks and even months until another truck comes along.
2. Concerning the effluents from the wetlands - little was actually coming out, most of it was evaporated,

It appears that loading of septage is intermittent. As you mentioned it varies from a few weeks to few months. For total management of septage for a particular volume what should be the no. and size of the units.
Constructed wetland based on septage has been implemented in AIT Bangkok. As per the results and reports ( ref. Faecal Sludge Management, SENDEC/IWA) "45% of the total liquid in the loaded sludge ended up as leachate, while 5% remained in the sludge layer, and 50% was lost to evapotranspiration). In general, leachate stops draining from the bed one to two days after loading." However, in your case little comes out in the form of leachate.
What could be reason for such nearly absence of leachate.

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Dear Pawan,

The reason is simply high rates of evapotranspiration. Because of the intermittent loading, the water is first filling pore space, and in order to maintain enough water for the plants, we were trying to keep the water level at a level just below the level of the original surface of the wetland. So, while that volume is filling up, nothing can come out. In addition, Adjumani has a rather dry savannah climate with day temperatures going up to 40 degrees C. In addition, there are frequent dry winds along the valley, where the facility is located. All that is firing up evapotranspiration. But of course, there is a difference between the dry and the rainy seasons. During the rains, leachates occur, and they are even essential for maintaining the plants. Otherwise, the plants would be suffering from salination of the soil.

In fact the best thing would probably be to allow the wetlands to drain during the rains, and to keep the water level in the wetlands up during the dry season. At the moment, however, these pipes at the outflow that allow to keep the water level up have been stolen. But ideally, that's the way it should be operated.

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Featured UserDec 2015

Hello Jim,

thanks for your interest! Concerning your questions:

1. The owner of the facility is the Adjumani Town Council. It was built and started up as a cooperation between GIZ and the Town Council. The operator is a local farmer and businessman who is also operating a cesspool emptying truck, and more recently something like a Gulper pit emptying machine.

2. Usually it's pit emptying trucks of 3 or 4 m3 capacity belonging to various owners (but first of all the one belonging to the operator himself), and, when it comes to the the Gulper machine, as far as I know, bins are used that are transported using a pickup truck. The large trucks are used for large latrines (like the ones of schools) or septic tanks, the Gulper/pickup technology is used predominantly for unlined home latrines.